The present invention relates to an apparatus for foaming under vacuum conditions, or, more simply, for “vacuum foaming”, wherein a chemically reactive polyurethane mixture is injected into the hollow walls of containers for refrigeration of foodstuffs, such as refrigerator cabinets for domestic and industrial use, freezers and the like, of the type disclosed in a previous international application WO 2010/094715, of the same applicant, representing the closer prior art.